A shrew, hunting insects
along a stream bank, slips into the icy water. It swims frantically
to reach shore, using all its energy to stay afloat.
Just as it appears the
small critter might make it, an almost imperceptible ripple appears.
And then the water explodes. The surface soon calms, but the shrew
is gone. Trout
Variations on this theme
in fishing lore, and there are plenty
of photos that prove this does actually occur.
But how frequently?
article appearing in the journal Ecology of Freshwater Fish
suggests this answer: more often than you think.
The paper by coauthors
Peter Lisi, Kale Bentley, Jonathan Armstrong and Daniel Schindler
documents the incidence of rainbow trout and grayling over
a 13-year period in the Wood River basin, part of the Bristol
Bay watershed in Alaska.
The Shrew Hatch
Its no secret that trout and other fish will eat small mammals.
A year ago, I reported on a rainbow
trout with 20 shrews in its stomach, documented during a
fish survey at Togiak
National Wildlife Refuge in Alaska. It remains one of the
most popular stories ever published on Cool Green Science.
Incidences like this
are often reported as isolated and opportunistic. Big trout will
gulp down a vole or shrew if one falls in, but mammals havent
been considered a major part of the diet.
This study documents
that rainbow trout and grayling actually key in on shrews
every two to three years when the mammals may be at peak abundance,
much as trout will focus on mayflies or caddis flies when
these insects hatch. The small mammals could provide important
nutritional value to fish.
In peak years, about
25 percent of rainbow trout and grayling larger than 12 inches
had eaten the small mammals.
Lisi, lead author of the study, was working on other fish
research while completing his doctorate under Daniel
Schindler at the University
of Washington (Lisi is now a post-doctoral fellow at the
The researchers kept
finding trout and grayling with shrews in their stomachs. They wondered
how often this actually occurred.
|Last years photo of a rainbow trout from Togiak National
Wildlife Refuge with 20 shrews in its stomach. This is not as
isolated an incident as many believe. Photo courtesty U.S. Fish
& Wildlife Service
It became a side
research project, something we did out of scientific curiosity,
Lisi says. How often do these mammals end up in the water?
How often do fish eat them? It turns out that every few years
in the Wood River basin, trout key in on shrews.
Shrews are insectivorous
mammals that are known to have boom and bust cycles. The
researchers speculate that in years of peak abundance, trout and
grayling eat them in significant numbers.
Fish are good at
selecting rich prey sources, says Lisi. Fly fishers
know this well. During a mayfly hatch, if you dont have the
fly, youre not going to catch anything, because trout
are all focused on a very specific insect.
How do the shrews
wind up in the water? Lisi notes that researchers can only speculate
at this point, especially since (unlike trout) shrews are under-studied
A couple of common
explanations can be ruled out.
In the Wood River basin,
the phenomenon does not appear to be associated with flooding, where
small mammals are washed into the stream with heavy rain.
There is a species of
shrew that swims and hunts aquatic prey, but this species
is not found in the Bristol Bay region.
In this case, the predation
may simply come down to abundant prey near the water.
Lisi notes that they
observed some years when shrews were abundant, and that it seemed
that during these times shrews were very active along stream banks.
The shrews are likely hunting insects and other prey.
Shrews have a high
metabolism, so they have to eat frequently, Lisi says.
When there are so many shrews, it might lead some to take
chances, to hunt close to the edge of the stream or even wade
into the stream for aquatic insects.
The researchers saw shrews
crossing very small streams, although some fishing guides have reported
that these animals dont swim very well, making them easier
prey for trout.
Given the incidence
of predation we saw, its highly doubtful any shrew could make
it across a large river, says Lisi.
One intriguing possibility
is that trout also pull shrews off banks, like the
orcas that attack seals on beaches in Argentina, or the
video-documented cases of catfish
lunging for pigeons along waters edge in France.
Do trout hunt prey in
this way? If youve seen it, Id love to hear about it.
Big Fish, Big
A grayling has to be at least 12 inches
before it can successfully feed on shrews. Photo: © Jonny
Not all rainbow trout
and grayling eat shrews. In part, this is simply because not all
trout and grayling can even fit a mammal into their mouths.
Lisi reports that it
takes a 12-inch grayling to eat a shrew. But not all 12-inch
grayling eat mammals.
Lisi reports that they
surveyed grayling in pools where a 12-inch fish was the largest
in the pool, and that fish had shrews in its stomach.
Other waters had pools
full of 12-inch fish but also contained some larger fish. In these
situations, it was only the larger fish that had grayling in their
stomach not the twelve inchers.
This suggests interesting
social dynamics going on here, says Lisi. The biggest
grayling are excluding the others from eating shrews, and capitalizing
on that resource. Often the largest fish had as many as six shrews
in their stomachs.
How often does this occur
outside Bristol Bay? There are a lot of reports of fish eating mammals,
eating multiple voles at Idahos Silver Creek, also
covered in a previous blog. In Mongolia,
taimen have been found with more than 40 lemmings in
their stomachs. Thats right: forty.
But even for highly studied
species like trout, theres a lot we dont know. Observations
by anglers can help a lot in understanding fish ecology and behavior.
Detailed scientific studies
occurring over a period of years offer the most valid information
particularly when those studies occur in a healthy, intact
ecosystem like Bristol
There are a number of
other studies coming out of Bristol Bay that could shed new light
on fish behavior and ecology. Ill report on more of these
findings in the coming months.
In the meantime, keep
those stories of what the trout eats coming.
The paper: Lisi,
P. J., Bentley, K. T., Armstrong, J. B. and Schindler, D. E. (2014),
Episodic predation of mammals by stream fishes in a boreal river
basin. Ecology of Freshwater Fish, 23: 622630. doi: 10.1111/eff.12117
Opinions expressed on
Cool Green Science and in any corresponding comments are the personal
opinions of the original authors and do not necessarily reflect
the views of The Nature Conservancy.
Miller is a senior science writer for the Conservancy. He
writes features and blogs about the conservation research being
conducted by the Conservancys 550 scientists. Matt previously
worked for nearly 11 years as director of communications for the
program. He has served on the national board of directors
of the Outdoor Writers Association of America, and has published
widely on conservation, nature and outdoor sports. He has held two
Coda fellowships, assisting conservation programs in Colombia and
Micronesia. An avid naturalist and outdoorsman, Matt has traveled
the world in search of wildlife and stories.